US10674253B2 - Apparatus and methods for monitoring a microphone - Google Patents
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- US10674253B2 US10674253B2 US15/935,428 US201815935428A US10674253B2 US 10674253 B2 US10674253 B2 US 10674253B2 US 201815935428 A US201815935428 A US 201815935428A US 10674253 B2 US10674253 B2 US 10674253B2
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
- H04R3/00—Circuits for transducers, loudspeakers or microphones
- H04R3/04—Circuits for transducers, loudspeakers or microphones for correcting frequency response
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- H—ELECTRICITY
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- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
- H04R1/00—Details of transducers, loudspeakers or microphones
- H04R1/20—Arrangements for obtaining desired frequency or directional characteristics
- H04R1/22—Arrangements for obtaining desired frequency or directional characteristics for obtaining desired frequency characteristic only
- H04R1/28—Transducer mountings or enclosures modified by provision of mechanical or acoustic impedances, e.g. resonator, damping means
- H04R1/2869—Reduction of undesired resonances, i.e. standing waves within enclosure, or of undesired vibrations, i.e. of the enclosure itself
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- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
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- H04R29/004—Monitoring arrangements; Testing arrangements for microphones
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- H04R1/00—Details of transducers, loudspeakers or microphones
- H04R1/20—Arrangements for obtaining desired frequency or directional characteristics
- H04R1/22—Arrangements for obtaining desired frequency or directional characteristics for obtaining desired frequency characteristic only
- H04R1/28—Transducer mountings or enclosures modified by provision of mechanical or acoustic impedances, e.g. resonator, damping means
- H04R1/2807—Enclosures comprising vibrating or resonating arrangements
- H04R1/2838—Enclosures comprising vibrating or resonating arrangements of the bandpass type
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- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
- H04R19/00—Electrostatic transducers
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
- H04R2201/00—Details of transducers, loudspeakers or microphones covered by H04R1/00 but not provided for in any of its subgroups
- H04R2201/003—Mems transducers or their use
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- H—ELECTRICITY
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- H04R2410/03—Reduction of intrinsic noise in microphones
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- H—ELECTRICITY
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- H04R3/00—Circuits for transducers, loudspeakers or microphones
Definitions
- This application relates to methods and apparatus for monitoring a microphone, such as a MEMS microphone, and especially to determining operating conditions for the microphone, in particular an acoustic port blockage status and/or temperature information and/or air resonance noise in the signal band.
- microphones for instance to allow for voice communication, for sound recording and/or for noise cancellation.
- Voice control is also becoming increasingly popular and to allow for the ability for voice control microphones may be provided on a range of devices.
- Ultrasonic communication is also of interest for machine-to-machine communication, requiring transducers for the transmitting and/or detecting of a transmitted ultrasonic signal.
- MEMS micro-electromechanical-system
- the microphone e.g. the MEMS microphone
- a noise reduction apparatus for a microphone device having an acoustic port comprising:
- the at least one characteristic of a resonance associated with the acoustic port of the microphone may comprise at least one a resonance frequency and a quality factor of said resonance. Additionally or alternatively the at least one characteristic of a resonance associated with the acoustic port of the microphone may comprise a first signal indicative of the relative magnitude of the resonance at a plurality of frequencies or within a plurality of frequency bins.
- the noise reduction block may be configured to process the microphone signal according to a first function, wherein said first function is based on said at least one characteristic of a resonance peak.
- the first function may be determined as an inverse function to the profile of the resonance peak.
- the noise reduction block may comprise a filter or filter bank.
- the spectrum peak detect block may comprise a conversion block configured to receive the microphone signal and output a spectrum signal corresponding to a frequency domain version of the microphone signal over at least a predefined frequency range.
- the conversion block may comprise a Fast Fourier Transform (FFT) block.
- FFT Fast Fourier Transform
- the spectrum peak detect block may comprise a filter for filtering the output of the FFT block to provide the spectrum signal.
- the spectrum peak detect block may comprise a filter bank configured to receive the microphone signal and output a spectrum signal comprising an indication of the signal amplitude at a plurality or frequencies or frequency bins over at least a predefined frequency range.
- the spectrum peak detect block may comprise a resonance frequency detection block configured to determine the resonance frequency from the spectrum signal.
- the resonance frequency detection block may be configured to identify the frequency with the highest signal magnitude within the predefined frequency range as the resonance frequency.
- the resonance frequency detection block may be configured to identify a signal magnitude range between the highest and lowest signal magnitudes within the predefined frequency range, determine a first threshold within the identified signal range and integrate the values of signal magnitude above the first threshold across the predefined frequency range to identify a first frequency at which the integral equal half of the total integral and identify said first frequency as the resonance frequency.
- the first threshold may be at a signal magnitude corresponding to a level of 50-70% of the signal magnitude range.
- the spectrum peak detect block may comprise a quality factor determination block configured to determine said quality factor from the spectrum signal.
- the quality factor determination block may be configured to determine a plurality of points associated with the resonance peak in the spectrum signal, to fit a curve to said determined points and to determine an indication of the quality factor from said curve.
- the quality factor determination block may be configured to determine a first point corresponding to a peak value of signal magnitude, and to identify second a third points a predetermined frequency above and below the first point respectively.
- the quality factor determination block may be configured to determine the width of the fitted curve at a predetermined magnitude interval below the peak magnitude as the quality factor.
- the first signal indicative of the relative magnitude of the resonance at a plurality of frequencies or within a plurality of frequency bins may be based on such a fitted curve.
- the apparatus may further comprise an activity detector configured to determine an indication of activity within the predefined frequency range.
- the activity detector may be configured to determine an indication of the difference between the current spectrum signal and at least one previous spectrum signal.
- the spectrum peak detect block may comprise a controller responsive to the activity detector. The controller may be configured such that if the activity is above an activity threshold the relevant spectrum signal is not used to determine an indication of resonance frequency or quality factor that is output from the spectrum peak detect block.
- the at least one characteristic of a resonance peak associated with the acoustic port of the microphone may comprise a resonance frequency and a quality factor of the resonance associated with the acoustic port.
- the apparatus may further comprise a condition monitoring block configured to determine any change in resonance frequency and quality factor with respect to respective reference values of resonance frequency and quality factor and to determine an operating condition for the microphone based on said detect changes.
- the operating condition may comprise at least one of: a blockage status of the acoustic port and an air temperature condition.
- the apparatus may be implemented as an integrated circuit.
- Aspects also relate to a microphone system comprising a first microphone and an apparatus as described in any of the variants above which is configured to receive a microphone signal from said first microphone.
- the electronic device comprising an apparatus as described in any of the variants above or the microphone system.
- the electronic device may be at least one of a: portable device; a battery powered device; a communications device; a mobile or cellular telephone; a smartphone device; a computing device; a laptop, notebook or tablet computer; a gaming device; a wearable device; a voice-controlled or voice activated device.
- aspects also relate to a method of noise reduction for a microphone device having an acoustic port, the method comprising:
- noise reduction circuit for a microphone comprising:
- a further aspect provides an apparatus for monitoring for blockage of an acoustic port of a microphone, the apparatus comprising:
- a further aspect provides an apparatus for monitoring for blockage of an acoustic port of a microphone, the apparatus comprising:
- a further aspect provides an apparatus for monitoring for blockage of an acoustic port of a microphone, the apparatus comprising:
- a further aspect provides an apparatus for monitoring a microphone device having an acoustic port, the apparatus comprising:
- a further aspect provides an apparatus for monitoring an operating temperature condition of a microphone device having an acoustic port, the apparatus comprising:
- a further aspect provides an apparatus for monitoring an air temperature condition of air within a microphone device, the apparatus comprising:
- a further aspect provides a temperature sensor comprising:
- FIG. 1 illustrates a packaged MEMS transducer having an associated acoustic port
- FIG. 2 illustrates an example of a microphone signal from a microphone having an acoustic port
- FIG. 3 illustrates a microphone monitoring apparatus for determining a blockage status and/or temperature condition
- FIG. 4 illustrates an example of a resonance spectrum peak detect block
- FIG. 5 illustrates the principles of determining a quality factor of a resonance peak according to one embodiment
- FIG. 6 illustrates a method of monitoring a microphone according to an embodiment
- FIG. 7 illustrates an idealised plot of noise floor for a microphone device
- FIG. 8 illustrates a microphone monitoring apparatus for noise reduction
- FIG. 9 illustrates an idealised plot of noise floor for the microphone monitoring apparatus of FIG. 8 .
- the operating conditions may, in some examples, comprise a blockage status of an acoustic port associated with the microphone device.
- the operating conditions may, in some examples, comprise a temperature condition for air within the microphone device, i.e. information about air temperature, e.g. an estimate of air temperature or information about a change in air temperature.
- the operating conditions may, in some examples, comprise information about noise due to the air resonance, at least within a signal band of interest, and in some embodiments the microphone signal may be processed to reduce the effect of any identified resonance noise within the signal band of interest, i.e. to improve the signal-to-noise ratio of the microphone signal.
- Embodiments of the present disclosure monitor a microphone signal generated by the microphone for a signal component related to a characteristic air resonance associated with the microphone device, i.e. a resonance associated the acoustic path from the outside world to the microphone transducer.
- the signal component related to such a resonance may be characterised and used to determine information about the operating conditions for the microphone and/or used to reduce or mitigate the noise impact of such resonance.
- a microphone device will comprise a transducer structure having an element that may be deflected by pressure waves, e.g. a flexible membrane, to produce a measurable change in electrical properties.
- the transducer structure is enclosed within a protective structure, e.g. a housing or package.
- a protective structure e.g. a housing or package.
- the microphone device thus comprises a cavity or volume formed within the package or housing and at least one acoustic port for acoustic communication with the external environment.
- Such a microphone structure will, in use, result in resonance within the cavity/acoustic port which will act upon the transducer structure.
- the microphone signal produced by the microphone will include a resonance peak associated with this air resonance.
- the characteristics of the air resonance, and hence the resonance peak in the microphone signal depend on the dimensions of the relevant air volumes of the microphone device, i.e. the dimensions of the acoustic port and cavity housing the transducer structure.
- the air resonance will, however, also depend on the air temperature.
- the microphone signal may be processed to reduce or eliminate such resonance components and thus improve the SNR (signal-to-noise ratio) of the microphone.
- FIG. 1 illustrates one example of a packaged MEMS microphone device 100 having an acoustic port.
- the microphone package comprises a MEMS microphone transducer 101 formed on a transducer substrate 102 .
- the transducer 101 typically comprises a membrane or diaphragm 103 that may be displaced by an incident pressure wave, e.g. an acoustic stimulus.
- acoustic shall be used to refer to any type of pressure wave that may propagate through air to deflect the membrane or diaphragm including, but not limited to, pressure waves at frequencies audible to a human, i.e. audible sound.
- the term acoustic shall be taken to include signals at ultrasonic frequencies.
- the membrane or diaphragm 103 forms or carries a first electrode (not illustrated separately in FIG. 1 ).
- the membrane or diaphragm 103 is supported relative to at a least one fixed back-plate structure 104 that has a second electrode (also not illustrated separately in FIG. 1 ).
- the moveable first electrode of the diaphragm 103 may tend to be displaced with respect to the fixed second electrode of the back-plate 104 .
- a measurement signal corresponding to the incident acoustic wave can be determined by monitoring the change in electrical properties, for example through measuring the change in capacitance due to deflection of the first electrode relative to the fixed second electrode.
- the MEMS transducer structure is thus typically coupled to biasing/read-out circuitry 105 and in some embodiments the transducer 101 and biasing/read-out circuitry 105 are formed as an integrated circuit on the same substrate 102 , i.e. the transducer 101 and biasing/read-out circuitry 105 are formed on a monolithic die. In other embodiments the biasing/read-out circuitry 105 may be formed on a separate die to the MEMS transducer structure, as indicated by the two vertical dashed lines within substrate 102 representing the edges of the respective dies in such an arrangement.
- the transducer 101 and circuitry 105 are typically packaged together within a housing or package 106 .
- the housing may comprise side wall portions 106 a , a lid portion 106 b and a base portion 106 c .
- at least some of these portions of the housing, i.e. package may be formed from continuous material, e.g. the side wall portion 106 a and lid portion 106 b may be formed from a shaped metal sheet and bonded to the base, i.e. substrate portion 106 c .
- the package may comprise separate components joined together to form the housing and may, for instance, be formed at least partly from printed circuit board (PCB) material such as FR4.
- PCB printed circuit board
- part of the transducer die 102 may form part of the housing structure.
- the housing 106 thus defines a cavity, or chamber, 107 within which the transducer 101 resides.
- the housing 106 has a housing port 108 , e.g. a hole or opening, which in this embodiment is illustrated as being in the cover, i.e. lid, of the package. It will be understood by those skilled in the art that the housing port 108 may alternatively be located in the substrate portion 106 c or the side wall portion 106 a .
- the housing port 108 may be an opening, somewhere suitable, in the housing 106 , although in some examples the housing port 108 may be sealed with some flexible membrane material to provide a substantially acoustically transparent port for transmission of acoustic waves but to provide a barrier, i.e. an environmental barrier, to ingress of moisture, dirt etc.
- the housing port 108 thus forms at least part of an acoustic port for the transmission of acoustic waves to the transducer 101 within the housing 106 .
- the housing port 108 may be coupled to a structure 109 , that at least partly defines an acoustic channel or waveguide for propagation of acoustic waves from a desired location, e.g. from the outside 111 of the host device, to cavity 107 within the microphone package 106 via the housing port 108 .
- structure 109 could be a gasket or the like.
- the channel 110 defined partly by guide structure 109 and partly by the housing port 108 can be seen as an acoustic port of the microphone device.
- the channel structure 110 may not be needed, for instance if the microphone package is located in a part of the host device that can readily receive acoustic waves of interest, in which case the acoustic port of the microphone may comprise the housing port alone. It will also be understood that other implementations for arranging a packaged microphone to receive acoustic signals of interest, but in general there will be an acoustic port associated with the packaged microphone.
- Such an acoustic system of an air volume or cavity 107 in fluid communication with the environment via an acoustic port 110 will exhibit a characteristic resonance, often referred to as the Helmholtz resonance.
- This resonance is a resonance of the air within the system and thus will be referred to herein as an air resonance.
- a Helmholtz resonance may be produced.
- air may flow past the entrance to the acoustic port 110 due to air currents or wind, acoustic stimuli (e.g. background noise) or movement of the host device through the air etc.
- a Helmholtz resonance may be spontaneously generated within the acoustic port 110 at various times.
- the Helmholtz resonance will have a characteristic frequency profile around a resonance frequency (the Helmholtz frequency f H ) which is related the dimensions of the acoustic system, e.g. the dimensions of the acoustic port 110 and internal cavity 107 of the microphone housing 106 .
- the acoustic path from the outside world to the transducer can be modelled as acoustic impedances, inductances and capacitances.
- acoustic impedance related to coupling of pressure waves from outside 111 the host device to the acoustic port, in series with an acoustic inductance due to the acoustic port 110 itself and a further acoustic impedance related to the interface between the acoustic port 110 and the cavity 107 .
- the cavity 107 also functions as an acoustic capacitance.
- This acoustic path can be modelled and behaves as a second order resonator.
- the resulting microphone signal from the MEMS transducer may therefore contain a signal component with the characteristic frequency profile associated with this Helmholtz resonance f H .
- FIG. 2 illustrates an example of a microphone signal from a packaged microphone having an acoustic port 110 , for example for a microphone package such as shown in FIG. 1 .
- a signal recorded in the absence of any specific acoustic stimulus being applied to the microphone, is indicative of the general background signal produced by the microphone.
- FIG. 2 illustrates a plot of signal magnitude (in dB in this case) against frequency for the microphone signal, i.e. is a spectrogram of the output signal from the microphone output from the readout circuitry 105 . It can clearly be seen that a peak 202 occurs in the plot, which in this instance had a peak frequency, i.e. a resonance frequency f H , of around 22 kHz.
- This peak 202 in the microphone signal is caused by the resonance associated with the acoustic system, i.e. acoustic port 110 leading to cavity 107 as discussed above.
- Analysis shows that, for a system such as illustrated in FIG. 1 , to first order the peak resonance frequency f H of this Helmholtz resonance can be estimated as:
- V the volume of cavity 107
- l the length of the acoustic port 110
- S A the area of the opening of the acoustic port 110
- c the speed of sound in the air in the acoustic port 110 and cavity 107 .
- the resonance frequency f H is proportional to the square root of the area S A of the acoustic port. If the area of the acoustic port 110 changes, for instance because the port becomes partly blocked, i.e. occluded, by some obstruction, the resonance frequency f H will change accordingly. In some embodiments therefore monitoring the frequency of the resonance peak that corresponds to the greatest magnitude of the signal component associated with the air resonance, i.e. the resonance frequency f H , can be used to determine the blockage state of the acoustic port 110 .
- microphone monitoring apparatus as shown in FIG. 3 may be used.
- FIG. 3 illustrates a microphone monitoring apparatus 300 according to an embodiment which may be used for monitoring a microphone with an associated acoustic port 110 , for instance such as the microphone arrangement illustrated in FIG. 1 .
- the microphone monitoring apparatus 300 is configured to receive microphone signals S MIC corresponding to the signals detected by the microphone transducer 101 .
- a microphone signal contains a signal component corresponding to a resonance associated with acoustic port 110 .
- the microphone signal S MIC is a digital signal corresponding to the microphone output signal but in some embodiments an analogue microphone signal could be received and converted to a digital signal for further processing.
- the microphone monitoring apparatus 300 comprises a spectrum peak detect block 301 for identifying a resonance peak associated with the acoustic port 110 in the microphone signal, i.e. an air resonance of the acoustic system.
- the spectrum peak detect block 301 is configured to determine one or more parameters related to the resonance peak and in the embodiment of FIG. 3 the spectral peak detect block 301 determines an indication of the resonance frequency f H .
- the spectrum peak detect block 301 also determines an indication of a quality factor Q H associated with the resonance peak as will be discussed in more detail below.
- a condition monitoring block 302 is configured to compare the one or more parameters related to the resonance peak to one or more reference parameters, e.g. reference values of resonance frequency f H and quality factor Q H , and to determine at least one operating condition for the microphone, for instance an indication of blockage status, i.e. whether acoustic port 110 is partly blocked, and/or an indication of air temperature or a change in air temperature.
- reference parameters e.g. reference values of resonance frequency f H and quality factor Q H
- the resonance frequency f H of the resonance peak depends partly on the area and length of the acoustic port 110 and thus a partial blockage of the acoustic port would change the area of the port and/or the length of the acoustic port with a consequent variation in the resonance frequency f H .
- equation (1) also shows that the resonance frequency f H depends on the speed of sound c.
- the speed of sound c will itself vary with temperature of the air in the acoustic port 110 as will be understood by one skilled in the art. For an ideal gas the speed of sound varies with the square root of temperature.
- the Helmholtz frequency f H is proportional to the speed of sound c and hence will vary with the square root of temperature.
- the Helmholtz frequency f H is also proportional to the square root of acoustic port 110 area S A .
- the opening of the acoustic port 110 were partly blocked, resulting in an effective reduction in the area of the acoustic port 110 of the order of 10% (with no change in air temperature)
- a certain change in the resonance frequency f H would be expected.
- the acoustic port 110 were to remain unblocked, so there is no change in effective area, but the temperature decreased by 10% (as determined in Kelvin) then the same change in resonance frequency would be expected to result. If one were purely to look at the resonance frequency f H of the resonance peak 220 then it may not be possible to differentiate a change in frequency arising from the acoustic port 110 being partly blocked from a change in air temperature.
- an indication of quality factor Q H of the resonance peak may also be determined.
- the quality factor Q H of a feature such as a resonance peak is an indication of the concentration or spread of energy of the resonance around the resonance frequency f H , i.e. an indication of how wide or narrow the resonance peak is in terms of frequency.
- a higher quality factor Q H means that most of the energy of the resonance is concentrated at the resonance frequency f H and the signal magnitude due to the resonance drops off quickly for other frequencies.
- a lower quality factor Q H means that frequencies near the peak resonance frequency f H may also exhibit some relatively significant signal magnitude.
- the quality factor Q H of a microphone arrangement such as illustrated in FIG. 1 may be given by:
- V, l and S A are the cavity, i.e. chamber, volume, port length and port area respectively as defined above for equation 1.
- the quality factor Q H of the resonance peak will vary with the area S A of the acoustic port 110 but the quality factor Q H is not temperature dependent.
- a change in air temperature without any change in acoustic port 110 area S A , would be expected to result in a change in the resonance frequency f H of the resonance peak but without a corresponding change in the quality factor Q H .
- partial closure, i.e. blocking, of the acoustic port resulting in a change in port area S A without a change in air temperature, would be expected to result in a change in both the resonance frequency f H of the resonance peak and also the quality factor Q H . Determining both the resonance frequency f H of the resonance peak, that is the frequency of the peak, and also the quality factor Q H thus allows for discrimination between changes in the resonance peak profile due to blockage in acoustic port 110 and changes due to temperature variation.
- FIG. 4 illustrates one example of the spectral peak detect block 301 according to an embodiment of the invention.
- the spectrum peak detect block 301 receives the microphone signal S MIC and a conversion block 401 converts the signal to a frequency domain version indicative of the signal magnitude, or power, for each frequency component, at least over a frequency range of interest, where any Helmholtz resonance may be expected.
- a Fast Fourier Transform (FFT) block 401 generates an FFT of the microphone signal S MIC , i.e. a spectrum signal indicative of the spectrum of the signals detected by the microphone transducer 101 .
- the microphone signal may be windowed, i.e. bandpass filtered, over a frequency range of interest in producing the spectrum signal.
- FFT Fast Fourier Transform
- the frequency range of interest may for instance, be based on the expected Helmholtz resonance frequency, assuming an unblocked acoustic port at standard temperature and pressure, with some defined range, e.g. ⁇ 30%.
- the output of the FFT 401 is filtered by filter block 402 to smooth the noise floor.
- a filter bank e.g. a bank of second-order resonators
- FFT block 401 may be omitted and filter 402 may comprise a filter bank as described.
- a resonance frequency detection block 403 receives the spectrum signal and determines the resonance frequency of the resonance peak, i.e. the frequency that corresponds to the peak signal magnitude.
- the resonance frequency of the resonance peak i.e. f H , may be determined.
- the resonance frequency detection block 403 is configured to determine the maximum A max and minimum A min signal magnitude values of the spectrum signal within a frequency range of interest, e.g. between two defined frequency values F 1 and F 2 .
- the resonance frequency may be expected to fall within a frequency range between 12 kHz and 35 kHz and thus the resonance frequency detection may be applied over this frequency range.
- a first threshold T 1 may be determined above which it is expected substantially all of the relevant signal contribution is due to the resonance peak.
- the signal magnitude values above the first threshold T 1 may then be integrated across the frequency range from F 1 to F 2 to determine a total integral value and also the value of a first frequency FR 1 where the integral corresponds to half of the total value.
- half of the total signal magnitude above the first threshold T 1 is at higher frequencies and half is at lower frequencies.
- the value of the first frequency FR 1 thus corresponds to the resonance frequency f H .
- the first threshold T 1 should be high enough so that any signal magnitude above the first threshold is expected to be part of the resonance peak due to the acoustic port 110 resonance, but should be low enough to include a reasonable width of the resonance peak.
- a threshold of the order of 50-70%, say about 60% of the signal magnitude range between A max and A min may be used in some embodiments.
- this resonance frequency f H associated with the acoustic port 110 may be used.
- the resonance frequency f H may simply be assumed to be the frequency with the highest signal magnitude in the frequency range of interest.
- the spectrum signal may be input to a suitably trained artificial neuron.
- the neuron may have been trained, for instance using FFT data and thresholds as described above.
- a quality factor determination block 404 may determine a quality factor Q H for the resonance peak.
- the quality factor block 404 may employ curve fitting to determine the quality factor Q H .
- the quality factor block 404 may, for instance, be configured to determine a plurality of points associated with the resonance peak in the filtered spectrum signal and then fit a curve, e.g. a parabolic curve, to said points. A quality factor Q H can then be determined from the fitted curve.
- the quality factor determination block 404 may determine a first point corresponding to the peak of the resonance peak, i.e. the frequency and signal magnitude at the peak. The quality factor determination block 404 may then step a defined frequency interval either side of this first point and determine the relevant signal magnitudes for those frequencies to provide second and third points.
- the relevant frequency step may be chosen according to the particular implementation and may, for instance by determined partly by the resolution of the spectrum signal, e.g. the resolution of the FFT.
- FIG. 5 illustrates an example of this approach.
- FIG. 5 illustrates an example spectrum signal 500 of signal magnitude against frequency over a frequency range of interest.
- a first point 501 is determined corresponding to the peak point of the resonance peak.
- the first point 501 may defined by determining the resonance frequency f H as described above and then determining the actual signal magnitude at this frequency f H .
- the first point 501 may be defined by looking for the frequency with the highest signal magnitude A max .
- the frequency that corresponds to the highest signal magnitude may be identified as the resonance frequency f H without applying the integration method described above.
- a second point 502 is defined by stepping up in frequency from f 1 by a defined amount ⁇ f and a third point is defined by stepping down in frequency from f 1 by the defined amount ⁇ f and in each case determining the relevant signal magnitude for those frequencies.
- the same frequency interval ⁇ f is used to define the second and third points but this does not have to be the case and other frequency interval could be applied and taken into account when applying the subsequent curve fitting.
- a parabolic curve 504 is then be defined which passes through all three points using any suitable known curve fitting process.
- the quality factor determination block 501 then estimate the quality factor of the resonant peak, Q H , as the width of the parabolic curve at a predetermined magnitude interval kdB below the maximum magnitude A max .
- the predetermined magnitude interval kdB may be of the order of 3 dB.
- the spectrum peak detect block 301 can thus determine values for both the resonance frequency f H and the quality factor Q H of the resonance peak.
- These values may be output from the spectrum peak detect block 301 to condition monitoring block 302 as illustrated in FIG. 3 .
- the spectrum peak detect block 301 thus has an activity detector 405 which is configured to monitor the filtered spectrum for any significant activity within the frequency band of interest for the Helmholtz resonance.
- the frequency profile of the resonance peak may not be expected to change much over time. Any temperature induced changes will tend to be gradual and if the acoustic port 110 becomes blocked the blockage may happen relatively suddenly but will likely remain in place for a period of time. However any external stimulus in the frequency band of interest may result in a continually changing spectral profile within the frequency band of interest.
- the activity detector 405 may thus compare the then current spectrum signal with a historic spectrum signal to determine the extent of any change in the relevant spectra.
- the activity detector 405 may thus receive the filtered FFT signal from filter 402 and may compare the current FFT signal to at least one stored previous FFT signal.
- the current FFT signal may be input to a buffer which stores a plurality of received FFT signals.
- the most recent FFT signal in the buffer may be subtracted term by term from the oldest FFT in the buffer, i.e. to determine any difference in signal magnitude at each frequency for the two FFTs.
- the absolute value of such difference may then be summed across the frequency range of interest to give an indication of activity in the frequency band of interest.
- An activity threshold T A may be applied to the determined sum, with data above the activity threshold T A being determined to be unreliable due to the level of activity in the frequency range. Any values of resonance frequency f H or quality factor Q H determined from a spectrum signal determined to be unreliable may be discarded.
- Activity such as speech may not result in much signal power in the frequency range of interest for a Helmholtz resonance of a ported microphone, i.e. a microphone with an acoustic port 110 , for instance in the example frequency range between 12 kHz and 35 kHz.
- a ported microphone i.e. a microphone with an acoustic port 110
- speech is unlikely to result in much activity in this frequency range of interest and the methods described above can typically be performed whilst the microphone is receiving speech.
- the presence of an acoustic stimulus with energy in the frequency band of interest may result in a rapidly changing spectrum in that range with a result that the determined activity level exceeds the activity threshold T A .
- Such data may therefore be discarded as being unreliable for detection of the parameters of the resonance peak associated with the acoustic port 110 .
- the activity detector 405 may thus provide a control signal to a controller 406 indicating whether there is any significant activity in the frequency range of interest and hence whether the determined values of resonance frequency f H and quality factor Q H are likely to be reliable or not. In the event that the activity detector indicates that there was no significant activity the controller may output the resonance frequency f H and quality factor Q H . In the event that the activity detector 405 does indicate that there was significant activity the controller 406 may not output any value or output the last previously reliable values.
- the values of resonance frequency f H and quality factor Q H are output, when deemed reliable, to the condition monitoring block 302 .
- the condition monitoring block 302 compares these parameters of the resonance peak to one or more reference parameter values.
- the reference values may correspond to a historically measured values, for instance one or more of the previously measured parameter values that may be stored as a reference value and/or a reference value could be determined as an average or some other combination of a plurality of previous values, i.e. historic values.
- the microphone may start producing a microphone output signal from which the input signal S MIC to the spectrum peak detect block 301 can be determined.
- the first values for resonance frequency f H and quality factor Q H to be determined may be used as initial reference values to which subsequently determined values may be compared.
- at least one initial reference value may be retrieved from a memory 303 as an initial reference. This stored reference value could be based on a value stored by condition monitoring block 302 when the microphone was last operated.
- the condition monitoring block 302 may be configured to store, in memory 303 , values based on the determined resonance frequency f H or quality factor Q H to be used as reference values for a subsequent use of the microphone.
- At least one stored reference value may be a stored value of resonance frequency f H or quality factor Q H which corresponds to a known status or condition of the microphone operation.
- values of resonance frequency f H and/or quality factor Q H could be determined as part of an initial factory test, carried out at a controlled temperature, and stored permanently in the memory 303 .
- Such a stored reference value may have been acquired in circumstances such that it is known that the acoustic port 110 is not blocked and such a stored reference value may thus be taken to correspond to an unblocked acoustic port 110 at a certain test temperature.
- at least one stored reference value may be a value which is modelled or calculated for the relevant microphone arrangement at certain conditions.
- values for the resonance frequency f H and quality factor Q H may thus be determined substantially continuously in use when the microphone is providing an output signal, or periodically at certain regular or irregular time intervals to monitor for any significant change in the values by comparison to one or more reference values. In the event of any significant change the status block may determine the cause of the change.
- the resonance frequency f H and the quality factor Q H are both substantially the same as some reference values. If the relevant reference values are previously determined values then, in that case, it can be assumed that the profile of the resonance peak has not substantially changed since those reference values were determined. Thus it can be assumed that there is no significant change in the relevant dimensions of the acoustic port 110 contributing to the Helmholtz resonance and that there is no change in the blockage status of the acoustic port 110 . If it is known or assumed that the reference values corresponds to the acoustic port 110 being unblocked then it can be determined that the acoustic port 110 remains unblocked. Further it can be determined that there has been no significant change in air temperature. If the relevant reference values correspond to a known temperature then it can be assumed that the air temperature corresponds to the known temperature.
- condition monitoring block 302 may be configured to simply determine that the air temperature has changed by determining a change in the resonance frequency f H that is not accompanied with a corresponding change in quality factor Q H .
- condition monitoring block may estimate the air temperature based on the determined value of resonance frequency f H .
- the quality factor Q H has not changed from a reference value known to correspond to an unblocked acoustic port 110 , then all change in the value of resonance frequency f H may be attributed to a change in air temperature.
- the temperature response for a particular microphone arrangement may be estimated, for instance according to:
- V, l and S A are the volume, length and port area of the ported microphone arrangement as discussed previously and ⁇ is the temperature in degrees Celsius.
- condition monitoring block may, in some embodiments be configured, in the event that the quality factor Q H is determined to correspond to a reference value indicative of an unblocked acoustic port 110 , determine an estimate of air temperature based on the present value of the resonance frequency f H .
- the resonance frequency f H is not the same as the reference value and the quality factor Q H of the resonant peak is also not the same as the reference quality factor.
- the blockage status of the acoustic port 110 has changed as the quality factor Q H is not the same as the reference quality factor. If it is known or assumed that the reference value corresponds to an unblocked acoustic port 110 then it may be assumed that the acoustic port 110 has become partly blocked, e.g. clogged.
- a blockage may occur relatively suddenly as the device with the microphone is placed onto a blocking object or vice versa or some debris enters the acoustic port 110 . Temperature changes may be more gradual. Therefore if there is a relatively sudden change in quality factor associated with a relatively sudden change in resonance frequency then it may be determined that a blockage has occurred (or been removed).
- a partly blocked acoustic port 110 may reduce the sensitivity of the microphone. If it is detected that an acoustic port 110 is blocked at least some aspect of the microphone operation or downstream processing may be adjusted to compensate or correct for the change in sensitivity. If the blocked state persists for some time an alert could be generated for the user of the device to check for any obstructions.
- Such an alert could, for example, be: an audible alert such as a bleep; a visual alert such as a warning message on the screen of the host device, such as a mobile phone for example; or a haptic alert such as a vibration of the host device or any combination of these three alerts.
- condition monitoring block 302 may receive indications of resonance frequency f H and quality factor Q H but may determine an estimate or temperature or temperature change only and may not report a separate blocked status (but may discount changes in resonance frequency f H due to blockage).
- condition monitoring block 302 may produce a status signal indicative of any change in blockage status and also indicative of a change in temperature and/or an estimate of the air temperature.
- the estimate of air temperature may be taken as an indication of ambient temperature.
- FIG. 6 illustrates a method of monitoring a microphone according to the present invention.
- the method comprises receiving a microphone signal S MIC from the microphone.
- the method comprises determining a resonance frequency f H associated with the acoustic port 110 in the microphone signal and also a quality factor Q H associated with the resonance peak.
- the air temperature may thus be known based on the reference value or in some embodiments the air temperature may be estimated, in step 605 , based on the determined resonance frequency.
- step 603 If however at step 603 it is determines that that the resonant peak is not the same as the reference peak, the method passes to step 606 at which it is determined if the quality factor Q H has varied, e.g. from a reference value of quality factor.
- the method may indicate at step 607 that there has been a change in temperature. As discussed above the method may then determine an estimate of temperature based on the resonance frequency.
- the quality factor Q H may be determined 608 that there has been some change in the blockage status of the acoustic port 110 associated with the microphone. If the reference quality factor corresponds to an unblocked port state then it may be signalled that the acoustic port 110 has become partly blocked.
- Embodiments of the disclosure thus provide apparatus and methods for monitoring a microphone having an acoustic port 110 .
- the methods and apparatus provide information on the air temperature of the microphone and/or on whether or not the acoustic port 110 of the microphone is blocked, by monitoring the frequency and quality factor of a resonance of the acoustic port 110 of the microphone.
- the characterisation of the signal component due to resonance may be used in processing the output signal to reduce or mitigate the effect of noise in the microphone signal.
- the resonance associated with the acoustic path from the environment of interest, e.g. the outside world, to the microphone transducer is an inherent resonance of the microphone structure and at least some signal component due to this resonance may be present whenever the microphone is receiving a signal of interest.
- the signal component due to resonance thus represents unwanted noise.
- this resonance may, in some implementations, lie mainly outside the audio band. Thus this resonance has mainly been ignored for standard audio applications.
- the effect of the resonance is that the transfer function between the pressure wave incident on the acoustic port 110 and the pressure experienced by the transducer 101 is non-linear with respect to frequency and high-frequency components are effectively amplified. The result is that acoustic signals in the near acoustic bandwidth may effectively be amplified, which in some instances may saturate the microphone. Also any acoustic noise is effectively amplified by the resonance decreasing the SNR performance.
- FIG. 7 illustrates an idealised plot of the signal response 701 from a microphone device, e.g. such as that illustrated in FIG. 1 , in the absence of any specific acoustic stimulus.
- the microphone signal may, as discussed above, include a signal component due to the Helmholtz resonance, i.e. a resonance peak 202 centred on a resonance frequency f H as discussed previously.
- FIG. 7 illustrates an upper cutoff for the audio band at 20 kHz. As mentioned previously the resonance frequency f H may be outside of the audio band but the resonance peak itself may extend partly into the audio band.
- the microphone signal from the MEMS transducer may be analysed to determine an operating condition of the microphone, wherein the operating condition is an indication of the noise contribution due to the resonance. This may be an indication of the noise contribution within the signal band of interest for the signals from the microphone, e.g. an audio band, or the total contribution of the noise due to resonance.
- the microphone signal may be processed to reduce or eliminate such resonance components and thus improve the SNR (signal-to-noise ratio) of the microphone.
- FIG. 8 illustrates a microphone monitoring apparatus 800 according to an embodiment.
- the microphone monitoring apparatus receives a microphone signal S MIC derived from the microphone transducer in the same manner as discussed above.
- the microphone signal may be input to spectrum peak detect block 301 which may determine one or more characteristics of the resonance peak in the same manner as discussed above.
- the spectrum peak detect block 301 may output the determined characteristics to a noise reduction block which applies processing to the microphone signal S MIC to produce a processed version S MIC_NR where at least some of the noise due to the resonance may be removed.
- the noise reduction block may comprise a filter or frequency processing block that is configured to effectively apply an inverse transfer function of amplitude to frequency to that determined for the resonance peak.
- the spectrum peak detect block 301 may determine the resonance frequency f H and/or the quality factor Q H and supply such values to the noise reduction block to process the received microphone signal S MIC to produce the noise reduced microphone signal S MIC_NR .
- the noise reduction block may be configured to apply a certain transfer function or filter based on the indication of resonance frequency and/or the indication of quality factor.
- a spectrum signal S(f) indicative of the amplitude of the resonance contribution with frequency could be determined and supplied to the noise reduction block 801 .
- the spectrum signal could, in some instance, comprise the parabolic curve determined by curve fitting by quality factor determination block 404 as discussed above in relation to FIG. 4 . Additionally or alternatively the spectrum signal could be a signal derived from the output of filter 402 at times when the activity detector 405 indicates no substantial activity.
- the noise reduction block may process the microphone signal S MIC to apply an inverse function to the determined variation of amplitude with frequency for the resonance peak.
- FIG. 9 illustrates that an inverse function 901 may be defined from the determined characteristic of the resonance peak 202 .
- the inverse function 901 is applied to the microphone signal S MIC and at least partly compensates for the resonance effects.
- FIG. 9 illustrates that the resultant processed signal 902 is, in the absence of any specific acoustic stimulus, substantially flat across the whole frequency range.
- FIG. 9 shows an idealised inverse transfer function but even a non-ideal function can help reduce the effects of resonance noise and provide a generally flatter frequency response across a desired frequency band.
- FIG. 9 illustrates the inverse function being applied across substantially the whole frequency range of the resonance peak. It would however be possible to apply an inverse function only over a frequency band of interest, for example just over the audio band if higher frequencies are not of interest.
- the microphone monitoring for noise reduction may be implemented with or without the monitoring discussed above to determine blockage status and/or a temperature condition.
- the microphone monitoring apparatus may include noise reduction and a condition monitoring block for determining blockage status and/or temperature status and in such embodiments the spectrum peak detect block may be shared as illustrated in FIG. 8 .
- Embodiments are particularly applicable to monitoring of MEMS microphones. However the principles may be applied to sensing signals from other types of microphone, electret microphones for example. Embodiments may be arranged as part of an audio and/or signal processing circuit, for instance an audio circuit which may be provided in a host device. Embodiments of the invention also relate to MEMS transducer circuits. A circuit according to an embodiment of the present invention may be implemented as an integrated circuit. A MEMS transducer may form part of the integrated circuit on a monolithic substrate or be connected to the integrated circuit in use.
- Embodiments may be implemented in a host device, especially a portable and/or battery powered host device such as a mobile telephone, an audio player, a video player, a PDA, a mobile computing platform such as a laptop computer or tablet and/or a games device for example.
- a host device especially a portable and/or battery powered host device such as a mobile telephone, an audio player, a video player, a PDA, a mobile computing platform such as a laptop computer or tablet and/or a games device for example.
- a functional block shall be used to refer to a functional unit or functional module of an apparatus or device, i.e. apparatus for performing a particular function.
- a functional block may be implemented at least partly by dedicated hardware components such as custom defined circuitry or programmable logic gates or the like. Additionally or alternatively a functional block may at least partly be implemented by one or more software processors or appropriate code running on one or more suitable processors.
- a processor may be a dedicated processor for performing the function or may be a shared processor that may also be used to perform other tasks and in some instances may be general purpose processor or the like.
- Components of a functional block may be co-located on the same circuit and/or implemented by the same processor or at least some components forming part of a functional block may be remote from one another and may be implemented on different integrated circuits or run on different processors. At least some components or processes forming part of one block may be shared with another functional block.
- a functional block may itself comprise other modules or functional units. The term module shall be used be taken to have the same meaning as the term block.
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Abstract
Description
-
- a spectrum peak detect block for receiving a microphone signal and determining, from the microphone signal, at least one characteristic of a resonance peak associated with the acoustic port of the microphone,
- a noise reduction block configured to process the microphone signal based on said at least one characteristic of a resonance associated with the acoustic port of the microphone so as to reduce noise in the processed microphone signal due to said resonance.
-
- receiving a microphone signal and determining, from the microphone signal, at least one characteristic of a resonance peak associated with the acoustic port of the microphone, and
- processing the microphone signal based on said at least one characteristic of a resonance associated with the acoustic port of the microphone so as to reduce noise in the processed microphone signal due to said resonance.
-
- a spectrum peak detect block for processing a microphone signal generated by the microphone to identify at least one characteristic of a spectral peak in the microphone signal due to a Helmholtz resonance; and
- a noise reduction circuit for processing the microphone signal based on said identified at least one characteristic of the spectral peak to reduce any noise due to the Helmholtz resonance in the processed microphone signal over at least a signal band of interest.
-
- a spectrum peak detect block for receiving a microphone signal and determining, from the microphone signal, a resonance frequency and a quality factor of a resonance associated with the acoustic port,
- a condition monitoring block configured to determine any change in resonance frequency and quality factor with respect to respective reference values of resonance frequency and quality factor and to determine a blockage status for the microphone based on said detect changes.
-
- a spectrum peak detect module configured to analyse a received microphone signal to determine a resonance frequency and a quality factor of a resonance associated with the acoustic port; and
- a condition monitoring block configured to determine a blockage status for the acoustic port of the microphone based on both said determined resonance frequency and said determined quality factor.
-
- a quality factor determining block for receiving a microphone signal and determining, from the microphone signal, a quality factor associated of a resonance associated with the acoustic port,
- a condition monitoring block configured determine a blockage status for the acoustic port of the microphone based on said determined quality factor.
-
- a spectrum peak detect block for receiving a microphone signal and determining, from the microphone signal, a resonance frequency and a quality factor of a resonance associated with the acoustic port,
- a condition monitoring block configured to determine any change in resonance frequency and quality factor with respect to respective reference values of resonance frequency and quality factor and to determine an operating condition for the microphone based on said detect changes;
- wherein the operating condition comprises at least one of: a blockage status of the acoustic port and an air temperature condition.
-
- a spectrum peak detect block for receiving a microphone signal and determining, from the microphone signal, a resonance frequency and a quality factor of a resonance associated with the acoustic port of the microphone,
- a condition monitoring block configured to determine any change in resonance frequency and quality factor with respect to respective reference values of resonance frequency and quality factor and to determine a temperature condition for the air temperature within the acoustic port based on said determined changes.
-
- a spectrum peak detect module configured to analyse a microphone signal to determine a resonance frequency and a quality factor of a resonance associated with the air within the microphone device; and
- a condition monitoring block configured to determine an air temperature condition for the air within the microphone device based on both said determined resonance frequency and said determined quality factor.
-
- a microphone device;
- a spectrum peak detect module for processing a microphone signal produced by the microphone device to identify a Helmholtz resonance peak frequency from the microphone signal; and
- a temperature estimation module for estimating air temperature based on said determined resonance Helmholtz resonance frequency.
where (to a first order) V is the volume of
where V, l and SA are the cavity, i.e. chamber, volume, port length and port area respectively as defined above for equation 1.
where V, l and SA are the volume, length and port area of the ported microphone arrangement as discussed previously and θ is the temperature in degrees Celsius.
Claims (20)
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Also Published As
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WO2018178640A1 (en) | 2018-10-04 |
CN110447240A (en) | 2019-11-12 |
CN110447240B (en) | 2021-06-25 |
US20180288523A1 (en) | 2018-10-04 |
GB2561021A (en) | 2018-10-03 |
GB2561021B (en) | 2019-09-18 |
GB201706707D0 (en) | 2017-06-14 |
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